For the comfort of the occupants in the passenger compartment, motor vehicles typically include dedicated air-conditioning systems and heating systems. The heating system includes a heater core located inside a heating, ventilating, and air conditioning (HVAC) module of the vehicle. The heater core is typically a liquid-to-air heat exchanger that supplies thermal energy to the passenger compartment for comfort heating. A heat transfer liquid, such as a glycol based coolant, conveys waste heat from an internal combustion engine to the heater core where the thermal energy from the heat transfer liquid is transferred to the ambient air flowing through the heater core to the passenger compartment. With the advent of greater efficiency internal combustion engines, hybrid vehicles having smaller internal combustion engines, and especially electrically driven vehicles, the amount of thermal energy available to provide comfort to occupants in the passenger compartment may not be adequate.
To provide supplemental heat to the passenger compartment for vehicles having smaller internal combustion engines, it is known to operate the air-conditioning system in heat pump mode. A typical motor vehicle air-conditioning system includes an evaporator located in the HVAC module and a condenser located in the front engine compartment exposed to outside ambient air. A compressor circulates a two-phase refrigerant through the evaporator where it expands into a low pressure vapor refrigerant by absorbing heat from the passenger compartment. After the low pressure vapor is compressed to a high pressure vapor by the compressor, the vapor phase refrigerant is transferred to the condenser where the high pressure vapor is condensed into a high pressure liquid refrigerant by releasing the heat to the ambient air. The liquid phase is returned to the evaporator through an expansion device which converts the high pressure liquid refrigerant to a low pressure mixture of liquid and vapor refrigerant to continue the cycle. By operating the air-conditioning system in heat pump mode, the refrigerant flow is reversed, in which case the condenser absorbs heat from the outside ambient air by evaporating the liquid phase refrigerant and the evaporator releases the heat to the passenger compartment by condensing the vapor phase refrigerant. One disadvantage to operating the air-conditioning system in heat pump mode, since the low pressure side of the system when used in air conditioning mode would become the high pressure side when used in heat pump mode, is the increase in system complexity due to the requirement of having to reinforce the refrigerant plumbing throughout the system by using thicker gage tubing and fittings. There is also the need to reinforce the evaporator to withstand the high pressure refrigerant, and to install an additional expansion device and receiver together with additional associated plumbing. Another known disadvantage of operating the system in heat pump mode is that in cooler climates, as the surface temperature of the condenser drop below 32° F., any moisture condensed on the surface of the condenser is subject to freezing, therefore resulting in reduced efficiency of the system and even potentially damaging the condenser.
Electric heaters are known to be used to provide supplemental heat to the passenger compartment for vehicles using the air-conditioning system as a heat pump. In the coldest of climates, it is known that operating the air-conditioning system in heat pump mode is ineffective; therefore, additional electric heaters are required. However, for hybrid and electrical vehicles, electrical heaters represent an increased current draw that significantly reduces the electric drive range.
Based on the foregoing, there is need for a heating system that provides supplementary heat to the passenger compartment of a motor vehicle that does not require reversing the refrigerant cycle of the air-conditioning system or detrimentally impact the electric driving range.